fix(dev/plugin)!: redo reverse proxy setup

[mkilpatrick]

Prior to this change, assets were imported relatively, also taking into account relativePathToPrefix. While this generally worked, there were other issues due to Vite's build optimizations: specifically module preloading and inline assets. Due to this we decided to take a new approach where assets are now imported absolutely and it's now up to the RP config to handle the assets. A working example of a Netlify setup for a RP at the subdirectory subfolder looks like this:

[[redirects]]
  from = "/subdirectory/assets/*"
  to = "https://rp-test.pagesprod.yextengtest.com/subdirectory/assets/:splat"
  status = 200
  force = true
  headers = {Host = "https://rp-test.pagesprod.yextengtest.com/"}
  
[[redirects]]
  from = "/subdirectory/*"
  to = "https://rp-test.pagesprod.yextengtest.com/:splat"
  status = 200
  force = true
  headers = {Host = "https://rp-test.pagesprod.yextengtest.com/"}
  
[[redirects]]
  from = "/subdirectory"
  to = "https://rp-test.pagesprod.yextengtest.com/index.html"
  status = 200
  force = true
  headers = {Host = "https://rp-test.pagesprod.yextengtest.com/"}

In order to properly configure the Vite side, a user must set the assetsDir in the vite.config.js (subdirectory/assets for the above example) or in the config.yaml (which will be globally supported in some upcoming PRs).

Other changes in the PR include a massive refactor of ProjectStructure in order to simplify its usage and is now being used everywhere, meaning that all filenames and folders are fully configurable.

[mancusi]

LGTM for the stuff i haven't yet commented on but still a few unresolved conversations